In the preferred application field of the present invention that was mentioned in the previous paragraph i.e. the power generation from ocean energy coming from the sea level fluctuations; there are energy extraction systems based on a buoy made of two parts, one of them floats freely following the sea surface, while the other has a reduced oscillation amplitude with respect to the first; this causes the relative movement to be reciprocating, making possible to extract energy from the buoy-sea system. The main characteristic of this movement is the presence of large forces and small velocities, which is a big difficulty for an efficient energy extracting device.
As it is well known, the waves are caused by the uninterrupted action of wind over large ocean surfaces. They travel for thousands of kilometres with almost no energy dissipation. Wave energy is characterised by being more predictable and concentrated than wind energy, so that the waves are an important energetic world resource. The estimated global wave power resource is 2 TW, which corresponds with the average world electric power consumption.
The traditional solution for extracting energy from systems characterised by a large force to speed ratio, is based on the use of hybrid hydraulic, mechanical and electrical systems. These systems are complex and their maintenance in a hostile environment may be very expensive. The availability of such a complex system, made of so many mechanical parts is necessarily lower, especially due to the reciprocating movement.
On the other hand, in the usual designs of switched reluctance electrical machines, the magnetic flux, created by the coils, closes after crossing only one air gap between the translator and the stator, which forces these components to have a large mass of ferromagnetic material for this purpose.
Stated in a more concrete way, in conventional switched reluctance machines, each coil only creates magnetic flux in one air gap, with the implication of not using a large amount of ferromagnetic material, with the derived consequences of larger cost and weight.
We can mention the existence of other electrical machines for direct energy generation from wave oscillations, which use rare earth permanent magnets in the translator, causing that the cost of the machine is rather high. Even in absence of an external grid, these machines have an open circuit voltage during operation, making its maintenance more dangerous and causing the partial disconnection of one of its phases, in case of failure, to be impossible. Other design of a linear machine has been proposed by the Uppsala University (Sweden), based on permanent magnets placed on an octagonal section and with distributed windings in the stator, a really more expensive and difficult to manufacture solution.
The European patent application EP 0 527 593 is relevant with respect to the present invention. This application describes an electric actuator motor comprising: a housing; a plurality of stator portions adapted within said housing, wherein each of said stator portions includes a plurality stator poles; a plurality of rotors movable between said stator portions; and electromagnetic means for moving said rotors between said stator portions. Nevertheless, this application fails to teach the possibility of including coils in the central stator.
The absence of the coils in the intermediate stators turns the topology of the machine into really non-scalable ad infinitum structure, due to the stray field of the coil-less central poles. Consequently, the ratio of aligned to non-aligned inductance increase at a much slower path than the number of translators. The use of coils in the central stators that we propose for the first time in this application creates a periodic structure which does not suffer from any problem of stray field in the central stators. Otherwise, the applicability of the multitranslator switched reluctance machine is limited to one central stator not equipped with coils. It is the introduction of the central coils which open the field of applications requiring hundred of kN of force in structures of limited length. In other words, the improvement is not only the additional magnetomotive force introduced in the central stators, but an improvement in the magnetic circuit. We have verified this statement in finite element models, obtaining substantial improvements with respect to the solution without coils in obtaining substantial improvements with respect to the solution without coils in the central stators.
Thus, the objective technical problem that the present invention has to solve is the reduction of the stray flux in the poles in order to increase the scalability of the motor structure ad infinitum and to increase the force to weight ratio.